Through the Fire and the Flames
What if the duel on mustafar had ended with Anakin losing his limbs but instead of being cooked to a scorching well done obi-wan showed Anakin mercy
"Its over Anakin!," Obi Wan yelled, "I have the high ground".
"You underestimate my power!" Anakin retorted.
"don't try it" Obi wan pleaded
However Anakin out of sheer spite and hubris attempted the near impossible jump. He was half a second to slow for Obi-wan's whirl to parry brought his lightsaber through Anakins left knee, then his right knee. His recovery to guard brought his blade through Anakin's right arm.
The crippled sith lord tumbled down the beach and dangerously close to the searing lava. Anakin knew it was only a matter of time before the flames reached him and possibly killed him.
Obi-Wan looked on at the site in front of him with extreme sadness in his eyes, in the force one would mistake him for a nervouse youngling and not the presteemed Jedi Master, or the best user of the Soresu form of lightsaber combat. The same combat that left his brother/best friend limbless and in a position to die.
At the same time he remembered all the times that Anakin had saved his life and while this monster was no longer the friend he had come to value and love as his brother there was still one last thing he could do for him.
Anakin lay there yelling out his proclamation to Obi-Wan, how he hated him and wished him to die. He felt the searing rise up his leg and prepared for the flames to swallow him hole, he instead felt as if he was being raised in the air before being dropped right where obi-wan stood. He also felt the flames being put out before it consumed him whole, he only had some minor burns that cauterized his wounds but nothing too serious that would require a life support or anything.
With a tear threatening to run down his eye, Obi-Wan whispered to Anakin " for old times sake my friend."
Anakin was in such a state of fury that he struggled to comprehend the Jedi Masters compassions. Rather he succumbed to the blissful darkness that was threatening his vision.
After he felt Anakin pass out, Obi-wan picked up his brothers lightsaber and walked away.